Airfoils of turbine blades and vanes (nozzles) of a gas turbine engine often require a complex cooling scheme in which cooling air flows through the airfoil and is then discharged through carefully configured cooling holes, for example, cooling slots at the trailing edge of the airfoil. The performance of a turbine blade is directly related to the ability to provide uniform cooling of its airfoil surfaces. Consequently, the control of cooling hole size and shape is critical in many turbine airfoil designs because the size and shape of the opening determine the amount of flow exiting a given opening, its distribution across the surface of the airfoil, and the overall flow distribution within the cooling circuit containing the openings. Other factors, such as backflow margin, are also affected by variations in opening size. In addition to conventional hole drilling techniques such as laser machining and electrical-discharge machining (EDM), complex advanced casting practices are typically used to yield airfoil castings with dimensionally correct openings in order to repeatably control opening size. Once cast, subsequent airfoil manufacturing operations must be performed such that cast-to-size openings are not processed through any operations that would significantly alter the dimensions of some or all of the openings.
Due to the severity of the operating environment of turbine airfoils, environmentally protective coatings are typically applied to these components when manufactured and also during repair. Platinum aluminide coatings are widely used to protect airfoils for this reason. Such coatings are produced by a multi-step process that includes plating platinum on surfaces requiring a protective coating, and then subsequently aluminizing the platinum plated surfaces by known methods such as pack cementation and noncontact vapor (gas phase deposition) techniques. As is apparent from the above, an environmental coating must not prevent the airfoil from meeting numerous operational requirements, including airflow requirements for the cooling air required to flow through the airfoil and exit through cooling holes and slots at the airfoil surface.
Current state of the art platinum plating baths deposit platinum on airfoils via electroplating, in which the airfoil serves as the cathode in the plating process. As a result of the unstable nature of precious metal-containing plating baths and the complex geometries of airfoils, plated material and/or precipitated material can accumulate in the surface openings of an airfoil, such as cast trailing edge slots and EDM diffusers of an air-cooled airfoil. Once present in the slots, these precious metal deposits can impact airflow by blistering, peeling, flaking or bridging inside the slot, or by increasing the effective platinum aluminide coating thickness in local areas or in the slot as a whole. These platinum-related perturbations act as flow restrictors for the slot, adversely impacting airflow and flow distribution through the airfoil. An airfoil with this condition is subject to rejection at the manufacturing level due to nonconforming air flow or, if in service, localized surface hot spots. Additionally, poor adhesion of the plating material can occur as a result of insufficient surface preparation of the substrate material in the slots, leading to subsequent spalling of the plating and coating during engine operation. Coating loss of this type changes the in-service flow distribution along the airfoil trailing edge, resulting in unpredicted airfoil distress.
In view of the above, various methods have been employed for preventing the deposition of platinum in the cooling holes of turbine airfoils. In addition to minimizing the change in cooling hole size and shape resulting from the plating operation, an additional benefit of preventing plating in the cooling holes is that the amount of platinum consumed by the plating operation is reduced. Generally, previous methods have involved mechanical masking, lacquering, taping and/or waxing, each of which is a conventional technique employed in the plating art for preventing the plating of specific surfaces on an article. However, each of these techniques has drawbacks when attempting to prevent plating in a surface opening. For example, mechanical masking and taping methods are generally limited to being suitable for large areas with generous transition zones between areas requiring plating and those that do not. Consequently, isolated small areas and/or exact areas where transitions between plated and unplated regions are critical do not lend themselves to mechanical masking or taping.
While lacquering can be effective for a wide range of uses, application is labor intensive. In addition, lacquer residues on adjacent surfaces can create plating defects, and additional manufacturing steps, such as ultrasonic cleaning and furnace burnout cycles, are required to remove lacquer after the plating operation. Ultrasonic cleaning techniques typically entail the use of ozone-depleting chemicals, and therefore are preferably avoided. Finally, while wax can be used to mask both external and internal surfaces during plating, methods by which wax is applied are imprecise and have been demonstrated to result in increased plating defects as a result of hydrocarbon contamination of the plating bath. As with lacquer, wax residues can be inadvertently left on surfaces that are intended for plating, causing the surfaces to be unplated or nonuniformly plated. Finally, after plating, components masked with wax must be processed through wax melting, ultrasonic cleaning and burnout cycles, at minimum, to remove all traces of wax from the component prior to subsequent processing.
Another drawback of lacquer and wax masking techniques is that, in order to control the thickness of the plating on an airfoil, the airfoil must typically be removed several times from the bath and weighed, providing an indication of the thickness of the platinum deposited. Once an airfoil is filled with lacquer or wax, the weight of the lacquer or wax must be accounted for when calculating the adequacy of the plating. However, loss of wax or lacquer during plating and plating solution trapped in the airfoil by the masking material inherently leads to false measurements and unintended plating thicknesses.
In view of the above, it can be seen that conventional masking techniques do not provide for a cost-effective, repeatable masking method for plating airfoils with detailed surface features on which plating is to be avoided. Accordingly, it would be desirable if an improved method were available for selectively plating an airfoil, and if such a method could prevent the deposition of material in the cooling holes and slots near the surface of an airfoil.